Saturday, September 18, 2010

The ability of phase-change materials to readily and swiftly transition between different phases has made them valuable as a low-power source of non-volatile or "flash" memory and data storage. Now an entire new class of phase-change materials has been discovered by researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley that could be applied to phase change random access memory (PCM) technologies and possibly optical data storage as well. The new phase-change materials -- nanocrystal alloys of a metal and semiconductor -- are called "BEANs," for binary eutectic-alloy nanostructures.

This schematic shows enthalpy curves sketched for the liquid, crystalline and amorphous phases of a new class of nanomaterials called “BEANs” for Binary Eutectic-Alloy Nanostructures. (Credit: Image courtesy of Daryl Chrzan)

"Phase changes in BEANs, switching them from crystalline to amorphous and back to crystalline states, can be induced in a matter of nanoseconds by electrical current, laser light or a combination of both," says Daryl Chrzan, a physicist who holds joint appointments with Berkeley Lab's Materials Sciences Division and UC Berkeley's Department of Materials Science and Engineering. "Working with germanium tin nanoparticles embedded in silica as our initial BEANs, we were able to stabilize both the solid and amorphous phases and could tune the kinetics of switching between the two simply by altering the composition."

Chrzan is the corresponding author on a paper reporting the results of this research which has been published in the journal NanoLetters titled "Embedded Binary Eutectic Alloy Nanostructures: A New Class of Phase Change Materials."

"What we have shown is that binary eutectic alloy nanostructures, such as quantum dots and nanowires, can serve as phase change materials," Chrzan says. "The key to the behavior we observed is the embedding of nanostructures within a matrix of nanoscale volumes. The presence of this nanostructure/matrix interface makes possible a rapid cooling that stabilizes the amorphous phase, and also enables us to tune the phase-change material's transformation kinetics."

A eutectic alloy is a metallic material that melts at the lowest possible temperature for its mix of constituents. The germanium tin compound is a eutectic alloy that has been considered by the investigators as a prototypical phase-change material because it can exist at room temperature in either a stable crystalline state or a metastable amorphous state. Chrzan and his colleagues found that when germanium tin nanocrystals were embedded within amorphous silica the nanocrystals formed a bilobed nanostructure that was half crystalline metallic and half crystalline semiconductor.

"Rapid cooling following pulsed laser melting stabilizes a metastable, amorphous, compositionally mixed phase state at room temperature, while moderate heating followed by slower cooling returns the nanocrystals to their initial bilobed crystalline state," Chrzan says. "The silica acts as a small and very clean test tube that confines the nanostructures so that the properties of the BEAN/silica interface are able to dictate the unique phase-change properties."

While they have not yet directly characterized the electronic transport properties of the bilobed and amorphous BEAN structures, from studies on related systems Chrzan and his colleagues expect that the transport as well as the optical properties of these two structures will be substantially different and that these difference will be tunable through composition alterations.

"In the amorphous alloyed state, we expect the BEAN to display normal, metallic conductivity," Chrzan says. "In the bilobed state, the BEAN will include one or more Schottky barriers that can be made to function as a diode. For purposes of data storage, the metallic conduction could signify a zero and a Schottky barrier could signify a one."

Chrzan and his colleagues are now investigating whether BEANs can sustain repeated phase-changes and whether the switching back and forth between the bilobed and amorphous structures can be incorporated into a wire geometry. They also want to model the flow of energy in the system and then use this modeling to tailor the light/current pulses for optimum phase-change properties.

The in-situ Transmission electron microscopy characterizations of the BEAN structures were carried out at Berkeley Lab's National Center for Electron Microscopy, one of the world's premier centers for electron microscopy and microcharacterization.

Thursday, September 16, 2010

(Nanowerk News) A Florida State University engineering professor's innovative research with nanomaterials could one day lead to a new generation of hydrogen fuel cells that are less expensive, smaller, lighter and more durable — advantages that might make them a viable option for widespread use in automobiles and in military and industrial technology.

Jim P. Zheng is a professor of electrical and computer engineering at the Florida A&M University-Florida State University College of Engineering, as well as a researcher at FSU's Center for Advanced Power Systems. Working with a material known as carbon nanotubes — essentially a form of carbon that is extraordinarily light and that easily conducts heat or electricity — he has designed a thin material, or membrane, that could reduce the need for expensive platinum components in hydrogen fuel cells.

"The driving issue involved in mass production of such fuel cells is one of cost," Zheng said. "Current hydrogen fuel cells use a platinum catalyst, making them too expensive to even consider producing on a large scale. However, by using carbon nanotube membranes, which are highly conductive and with unique properties, it might be possible to reduce the amount of platinum that is required. And since the membrane is thinner and lighter than current components, the fuel cell can be smaller and yet still provide the same amount of power."

Known as polymer electrolyte membrane fuel cells, or PEMFCs, this technology was initially developed for military and spacecraft applications at GE. To date, the technology has been extended to a wider scope of applications, with the potential to power a range of devices from mobile phones and laptops to cars, buses, boats, houses and even spacecraft.

FSU Vice President for Research Kirby Kemper emphasized the importance of energy research such as Zheng's at a time when the many economic, environmental and national-security issues related to the United States' dependence on oil make headlines every day.

(Nanowerk News) Manipulating materials with rotating quantum particles: a team from the University of Antwerp and TU Vienna (Professor Peter Schattschneider, Institute of Solid State Physics) has succeeded in producing what are known as vortex beams: rotating electron beams, which make it possible to investigate the magnetic properties of materials. In the future, it may even be possible to manipulate the tiniest components in a targeted manner and set them in rotation. The physicists report on this breakthrough in electron physics and its application in the current edition of Nature

Rotating current: the quantum tornado

Electron beams have been used to analyse materials for some time now – for example in electron microscopes. For the most part, the beams' rotation does not affect this analysis. In classical physics, an electron current in a vacuum does not have any orbital angular momentum. In quantum mechanics, however, the electrons must be envisaged as a wavelike current – which can rotate as a whole about its propagation direction, similar to the air flow in a tornado.

A flat wave (left) meets the specially shaped grid screen, which converts the electron beam into right-rotating and left-rotating vortex beams (top and bottom), and a middle beam that does not rotate. Similar to in a tornado, the rotation of the electron current is low internally

Vortex light beams have been used in optics for some time (for example, as optical tweezers for manipulating small particles). Vortex beams made from electrons also offer many new possibilities for managing nanoparticles or measuring angular momentum-related parameters. However, there were previously no really efficient methods of producing them. "When I was working on an idea of how these beams could be technically produced, it emerged that colleagues from Antwerp had had the same idea", explains Prof Schattschneider. "We therefore decided to pursue the project together: Antwerp had progressed further with the production and Vienna came up with a suggestion for the first application."

The trick with the screen

The production of vortex electron beams was made possible with the help of a grid-like screen cut from platinum foil. When it passes through the platinum screen, the electron beam is diffracted in a similar way to light beams when they pass through a fine grid. The shape of this screen, which measures only a few millionths of a metre, was specifically calculated so that a flat incident electron wave is converted into vortex beams. Right-rotating and left-rotating vortex beams are thus formed behind the grid and in the middle there is a conventional electron beam that does not rotate.

If the electrons are used to irradiate a material which for its part also influences the angular momentum of the electrons, and if the electrons are subsequently directed through the made-to-measure platinum screen, then, after this, either the right-rotating or the left-rotating vortex beam will be more intense. "This enables us to investigate processes affected by angular momentum in nanomaterials much more precisely than was previously possible", explains Prof Schattschneider.

Better than science fiction

The physicist, who also occasionally writes science fiction, does not find it hard to imagine more exotic applications for the vortex beams: "These electron beams could be used in a targeted way to set tiny wheels in motion on a microscopic motor. Also, the magnetic field of the rotating electrons could be used in the tiniest length scales", Schattschneider speculates. Even applications in data transfer (quantum cryptography) and quantum computers are feasible.

Source: Vienna University of Technologyhttp://www.nanowerk.com/news/newsid=18071.php

In the latest issue of NANO magazine, we look at many applications of nanotechnology to our everyday lives, and its promise for the future. This issue takes a look at how nanotechnology is being addressed in architecture, interior design, biomimetics, education, health and education.

Architecture, construction and Interior Design

Nanotechnology in architecture is addressed compelling by Sylvia Leydecker in this issue. She states that innovation-driven materials and products are critical in achieving green construction, which is now at the forefront of much architectural debate. Ms Leydecker believes that nanomaterials have a huge potential in this area, which is yet to be realised, as architects have not yet engaged fully with what is available.

Following on from a plea that architects become more acquainted with nanotechnology, the Decker Yeadon agency in New York has come up with new concepts based on nanotechnology that could shape the future of homes and offices. They so convinced by its benefits that they have just invested in making Buckypaper, a new material which has an electrically conductive coating of multi-walled nanotubes.

Biomimetics

The emphasis on nanoscience and nanotechnology since the early 1990s has provided a significant impetus in mimicking nature, using nanofabrication techniques for commercial applications. Bharat Bhusan takes us on a tour of the natural world and some of its attributes that are leading to new products using biomimetics. George Whitesides

The subject of this month's interview is Harvard Professor George Whitesides. Professor Whitesides is not only successful as an academic, but is also named on over 50 patents. A lifetime of knowledge and experience has led him to a profound understanding of what society needs from science. His view is that, where science thrives on complexity, and unexpected outcomes, society needs simplicity allied to function.

Education and Ethics

One way to improve understanding of nanotechnology is by engaging young people in dialogue about its ethical, legal and social aspects is needed. NANOYOU (Nano for Youth) is a project funded by the European Commission's Seventh Framework Programme that aims to achieve this through an appealing variety of media, games, role playing and other interactions.

The ethical debate on nanotechnology is an exciting one, which poses many complex questions -such as how we perceive nature, as opposed to artefact; the possible redefinition of the norms of health and disease; the likelihood of Transhumanism; the fair distribution of the benefits of nanotechnology; and scientists' responsibility for the consequences of technological innovations. Marc Pavlopoulos explores how we can ask the right questions, and the surprising ways in which society adopts a new technology.

Nanomedicine and the ageing population

This month's article on nanomedicine by Ottilia Saxl, explores the broader issues of how nanotechnology can provide important benefits to an ageing population, in terms of prolonging independence and quality of life for as long as possible, while reducing costs.

The country profile this month is Brazil. Brazil may have been a little later in getting to grips with the potential of nanotechnology, but investment and strong policies linking science and industry are reaping the benefits. Jos d'Albuquerque e Castro who has been involved in nanotechnology in Brazil, both from within University and Government, gives an all-round perspective on the state of the technology and where it is headed.

Friday, September 10, 2010

ScienceDaily (2010) — Creating catalysts that can operate efficiently and last a long time is a big barrier to taking fuel-cell technology from the lab bench to the assembly line. The precious metal platinum has been the choice for many researchers, but platinum has two major downsides: It is expensive, and it breaks down over time in fuel-cell reactions.

In a new study, chemists at Brown University report a promising advance. They have created a unique core and shell nanoparticle that uses far less platinum yet performs more efficiently and lasts longer than commercially available pure-platinum catalysts at the cathode end of fuel-cell reactions.

The chemistry known as oxygen reduction reaction takes place at the fuel cell's cathode, creating water as its only waste, rather than the global-warming carbon dioxide produced by internal combustion systems. The cathode is also where up to 40 percent of a fuel cell's efficiency is lost, so "this is a crucial step in making fuel cells a more competitive technology with internal combustion engines and batteries," said Shouheng Sun, professor of chemistry at Brown and co-author of the paper in the Journal of the American Chemical Society.

The research team, which includes Brown graduate student and co-author Vismadeb Mazumder and researchers from Oak Ridge National Laboratory in Tennessee, created a five-nanometer palladium (Pd) core and encircled it with a shell consisting of iron and platinum (FePt). The trick, Mazumder said, was in molding a shell that would retain its shape and require the smallest amount of platinum to pull off an efficient reaction. The team created the iron-platinum shell by decomposing iron pentacarbonyl [Fe(CO)5] and reducing platinum acetylacetonate [Pt(acac)2], a technique Sun first reported in a 2000 Science paper. The result was a shell that uses only 30 percent platinum, although the researchers say they expect they will be able to make thinner shells and use even less platinum.

"If we don't use iron pentacarbonyl, then the platinum doesn't form on the (palladium) core," Mazumder said.

The researchers demonstrated for the first time that they could consistently produce the unique core-shell structures. In laboratory tests, the palladium/iron-platinum nanoparticles generated 12 times more current than commercially available pure-platinum catalysts at the same catalyst weight. The output also remained consistent over 10,000 cycles, at least ten times longer than commercially available platinum models that begin to deteriorate after 1,000 cycles.

The team created iron-platinum shells that varied in width from one to three nanometers. In lab tests, the group found the one-nanometer shells performed best.

"This is a very good demonstration that catalysts with a core and a shell can be made readily in half-gram quantities in the lab, they're active, and they last," Mazumder said. "The next step is to scale them up for commercial use, and we are confident we'll be able to do that."

Mazumder and Sun are studying why the palladium core increases the catalytic abilities of iron platinum, although they think it has something to do with the transfer of electrons between the core and shell metals. To that end, they are trying to use a chemically more active metal than palladium as the core to confirm the transfer of electrons in the core-shell arrangement and its importance to the catalyst's function.

Miaofang Chi and Karren More at the Oak Ridge Laboratory also contributed to the paper. The U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy funded the research as part of its Fuel Cell Technologies Program.

Tuesday, September 07, 2010

ScienceDaily (Sep. 4, 2010) — The modern world -- with its ubiquitous electronic devices and electrical power -- can trace its lineage directly to the discovery, less than two centuries ago, of the link between electricity and magnetism. But while engineers have harnessed electromagnetic forces on a global scale, physicists still struggle to describe the dance between electrons that creates magnetic fields.

Two theoretical physicists from Rice University are reporting initial success in that area in a new paper in the Proceedings of the National Academy of Sciences. Their new conceptual model, which was created to learn more about the quantum quirks of high-temperature superconductors and other high-tech materials, has also proven useful in describing the origins of ferromagnetism -- the everyday "magnetism" of compass needles and refrigerator magnets.

"As a theorist, you strive to have exact solutions, and even though our new model is purely theoretical, it does produce results that match what's observed in the real world," said Rice physicist Qimiao Si, the lead author of the paper. "In that sense, it is reassuring to have designed a model system in which ferromagnetism is allowed."

Ferromagnets are what most people think of as magnets. They're the permanently magnetic materials that keep notes stuck to refrigerators the world over. Scientists have long understood the large-scale workings of ferromagnets, which can be described theoretically from a coarse-grained perspective. But at a deeper, fine-grained level -- down at the scale of atoms and electrons -- the origins of ferromagnetism remain fuzzy.

"When we started on this project, we were aware of the surprising lack of theoretical progress that had been made on metallic ferromagnetism," Si said. "Even a seemingly simple question, like why an everyday refrigerator magnet forms out of electrons that interact with each other, has no rigorous answer."

Si and graduate student Seiji Yamamoto's interest in the foundations of ferromagnetism stemmed from the study of materials that were far from ordinary.

Si's specialty is an area of condensed matter physics that grew out of the discovery more than 20 years ago of high-temperature superconductivity. In 2001, Si offered a new theory to explain the behavior of the class of materials that includes high-temperature superconductors. This class of materials -- known as "quantum correlated matter" -- also includes more than 10 known types of ferromagnetic composites.

Si's 2001 theory and his subsequent work have aimed to explain the experimentally observed behavior of quantum-correlated materials based upon the strangely correlated interplay between electrons that goes on inside them. In particular, he focuses on the correlated electron effect that occur as the materials approach a "quantum critical point," a tipping point that's the quantum equivalent of the abrupt solid-to-liquid change that occurs when ice melts.

The quantum critical point that plays a key role in high-temperature superconductivity is the tipping point that marks a shift to antiferromagnetism, a magnetic state that has markedly different subatomic characteristics from ferromagnetism. Because of the key role in high-temperature superconductivity, most studies in the field have focused on antiferromagnetism. In contrast, ferromagnetism -- the more familiar, everyday form of magnetism -- has received much less attention theoretically in quantum-correlated materials.

"So our initial theoretical question was, 'What would happen, in terms of correlated electron effects, when a ferromagnetic material moves through one of these quantum tipping points?" said Yamamoto, who is now a postdoctoral researcher at the National High Magnetic Field Laboratory in Tallahassee, Fla..

To carry out this thought experiment, Si and Yamamoto created a model system that idealizes what exists in nature. Their jumping off point was a well-studied phenomenon known as the Kondo effect -- which also has its roots in quantum magnetic effects. Based on what they knew of this effect, they created a model of a "Kondo lattice," a fine-grained mesh of electrons that behaved like those that had been observed in Kondo studies of real-world materials.

Si and Yamamoto were able to use the model to provide a rigorous answer about the fine-grained origins of metallic ferromagnetism. Furthermore, the ferromagnetic state that was predicted by the model turned out to have quantum properties that closely resemble those observed experimentally in heavy fermion ferromagnets.

"The model is useful because it allows us to predict how real-world materials might behave under a specific set of circumstances," Yamamoto said. "And, in fact, we have been able to use it to explain experimental observations on heavy fermion metals, including both the antiferromagnets as well as the less well understood ferromagnetic materials."